This application for an ADAMHA Research Scientist Award focuses on the interactions between the neuroendocrine controls of reproduction and energy balance. Regulation of energy balance and reproductive function are closely related because successful reproduction is critically dependent upon an adequate supply of energy. When food is scarce or when the energy demands of other physiological processes (e.g., exercise, thermoregulation, growth) are excessive, animals will defer reproduction in favor of activities that promote individual survival. One aspect of this research program examines the physiological mechanisms by which the level of nutrition controls reproduction, using Syrian hamsters as a model. Using selective inhibitors of glycolysis and fatty acid oxidation, we have shown that reproduction is sensitive to the general availability of oxidizable metabolic fuels, rather than to any single fuel or to body fat content. Future work will focus on determining which sites monitor the availability of metabolic fuels and how these sensors communicate with the neural mechanisms controlling reproductive behaviors and gonadotropin synthesis/release. However, reproduction is not merely the passive recipient of whatever metabolic fuels are left after other processes have taken their share of the available calories. Reproductive activity has profound effects on the behavioral and physiological processes controlling the procurement, ingestion, partitioning, storage, and expenditure of metabolic fuels via changes in the circulating levels of gonadal hormones. A second component of this research program studies the effects of gonadal hormones on regulation of energy balance in laboratory rodents. This work has shown that gonadal hormones act on a variety of neural and non-neural target tissues to affect both regulatory behaviors (e., food intake, voluntary exercise) and metabolic processes (e.g., gatekeeper enzymes, thermogenesis). Ongoing research includes studies directed at the effects of progesterone on regulatory behaviors and the partitioning of metabolic fuels and the role of sympathetic nerves in gonadal regulation of energy balance. Numerous temperate-zone species exhibit dramatic seasonal cycles in reproduction and energy balance, particularly body fat stores. In many mammals, it is the photoperiod (day length) that signals seasonal change and drives these physiological and behavioral fluctuations. The third focus of this research program is directed at elucidating the neuroendocrine mechanisms for seasonal body weight cycles in Syrian hamsters. In photoperiodic species, the pineal gland acts as a neuroendocrine transducer, converting day length information to an endocrine signal via nocturnal secretion of the hormone, melatonin. Work in progress is attempting to identify the physiological mediators of the effects of photoperiod and melatonin on energy balance.